The present invention relates to apparatus and method for the selective wetting of a mixture of particles with a selective wetting agent, such as a surface-active agent. More particularly, this invention relates to an apparatus and method for selectively wetting select particles of a mixture of particles with a surface-active agent whereby particles are selectively wetted (coated) with the surface-active agent in an amount relative to the amount of a selected component within the particles with which the surface-active reagent is reactive. The particles containing such a selected component are coated with the surface-active agent in amounts dependent upon the amount of selected component to the substantial exclusion of coating the particles containing essentially none of the component.
Selective wetting of particles is required for many operations for separating a mixture of particles. In particular, the selective wetting of particles is practiced in the mining and ore beneficiating art for separating minerals from the ore in which it is contained. Some methods used to separate a mineral from other components within a mineral ore rely upon the differences in the surface chemical properties of the components in the ore. The surface chemical properties of a particle of ore depend upon components present in that particle of ore. As the particles contain varying compositions, the particles can be distinguished and thereby separated according to their composition through use of the surface chemical properties of the individual particles.
In order to make use of the differences in the surface chemical properties of the particles of ore, it is often necessary to contact the mixture of particles with a surface-active agent that will selectively react with some of the particles due to the selectivity of the agent in distinguishing between surface chemical properties. "Reaction" may be chemical, physical or a hybrid of the two. The process of contacting a mixture of particles with a surface-active agent is referred to in the art as "conditioning." Methods of particle separation in which it is necessary to condition the particles include methods such as flotation separation and optical separation. In flotation separation the particles to be separated are conditioned with a flotation agent. The flotation agent coats the ore particles with which it is reactive either with a coating which makes the particles float or with a coating which suppresses flotation while other particles are floated. For example, some flotation agents impart a hydrophobic coating to particles which coating assists in floating the particles to enable separation.
In an optical separation process such as a fluorescent separation method, a mixture of particles to be separated is conditioned with an aqueous dispersion of a suitable surface-active agent and a fluorescent material, e.g., a dye. The selectively coated particles exhibit fluorescence when exposed to actinic radiation, such as ultraviolet light. The particles which are poorly coated with the surface-active agent and fluorescent dye combination do not exhibit significant fluorescence when exposed to actinic radiation. Based on the difference in fluorescence, coated particles can be separated from those which are poorly coated in an apparatus as described in Mathews U.S. Pat. Nos. 3,472,375; 3,356,211; and 3,722,676, all of which are incorporated herein by this reference.
Methods currently employed for conditioning particles generally consist of either one of two conditioning methods. One current method practiced is immersing the mixture of particles to be separated in a fluid bed containing the surface-active agent. A disadvantage of such a method is that a large reservoir of the surface-active agent must be maintained in order to continuously wet the mixture of particles as the particles move through the fluid bed. Another disadvantage of such a method is that the particles may not be coated on their surface which is in contact with the transfer device conveying the particles through the fluid bed. This partial coating of the particles can inhibit the efficient separation of the particles from the mixture. For example, if an optical separation technique is employed wherein the residence time for a particle in a detection zone is relatively short, the non-coated surface of a particle can be presented to the detection means. The particle that is only partially coated can, therefore, be undetected resulting in inefficient separation. In the flotation separation process, if a surface of a particle is not coated with the flotation agent, the particle may not acquire the necessary change in surface hydrophobicity to bring about flotation or suppression of that particle. Therefore, such a particle can escape separation.
A second method of wetting a mixture of particles is by spraying the surface-active agent onto the mixture of particles as the particles are transported on a moving belt or a vibrating screen by the sprayer. In this method of wetting the particles there exists the possibility that the particles will be nonuniformly coated. The surface of the particles that are in contact with the belt may not be coated. Even if the belt were vibrating some surface on the particles may not be coated by the sprayer. As the particles are also in contact with a surface, there can be smearing of the surface-active agent on the particle and on the particles that are not normally surface-chemically-active with the reagent. Thus, separation based upon the coating imparted to the particles can be inefficient.